The present invention relates to a polymer dispersion liquid crystal panel forming optical images as changes in the light scattering condition; to a manufacturing method for such a liquid crystal panel; and to a projection display device for enlarging and projecting to a screen the images displayed on this liquid crystal panel.
Interest in large-screen display devices has grown significantly in recent years for applications such as home theater and business presentations. While many different types of projection devices using light valves have been proposed over the years, liquid crystal projection televisions whereby images displayed on a small liquid crystal panel are enlarged and projected using a projection lens and/or other optics to a large screen have more recently become available.
Images are displayed on liquid crystal panels primarily by electrically changing the optical characteristics of the liquid crystals. There are many different methods of accomplishing this based on a variety of operating principles. Twisted nematic (TN) liquid crystal panels used in currently available liquid crystal projection display devices use the changes in the optical rotatory power of the liquid crystals effected by varying the field strength. The drawback to this method is that TN liquid crystal panels require a polarizing plate on both the incidence and emission sides for light modulation, and these polarizing plates lower the efficiency of the device for light utilization.
Methods using the light scattering phenomenon of the liquid crystals can be used to control light without using polarizing plates. Examples of liquid crystal panels whereby optical images are formed by changing the light scattering state of the liquid crystals include phase change (PC), dynamic scattering (DSM), the polymer dispersion liquid crystals. Due to demand for improved image brightness, polymer dispersion liquid crystal panels such as described in U.S. Pat. No. 4,435,047 are being actively researched.
Polymer dispersion liquid crystals are described briefly below. Polymer dispersion liquid crystals can be divided into two major types according to the dispersion state of the liquid crystals and polymer. In one type, drop-shaped liquid crystals are dispersed in a polymer substance, and the liquid crystals are present in the polymer substance in a discontinuous state; this type of liquid crystal is referred to as a "PDLC" (polymer dispersion liquid crystal) below. In the other type, a network of polymer is laid through the liquid crystal layer, resulting in a structure similar to a sponge impregnated with liquid crystals. The liquid crystals in this structure are not drop-shaped, and are continuous throughout the structure; this type is referred to as a "PNLC" (polymer network liquid crystal) below. In both types of liquid crystal panels, images are displayed by controlling the light scattering and transmission states of the liquid crystals. Note that the present invention is described by way of example using primarily a PDLC. Provided that the term PDLC herein used is to be understood as including not only the polymer dispersion liquid crystal material but also the polymer network liquid crystal material as well.
Insofar as the resin is transparent, the polymer matrix in this type of polymer dispersion liquid crystal layer can basically be either a thermoplastic or thermosetting resin. Ultraviolet-setting resins are the simplest and offer good performance, and are therefore most commonly used. This is because the same manufacturing method used for TN liquid crystal panels can be applied without modifications.
To manufacture conventional liquid crystal panels, the specified electrode pattern is first formed on the two top and bottom circuit boards, and these two circuit boards are laminated together with the corresponding electrodes in opposition. In the lamination process, a spacer having a uniform, predetermined grain size is sandwiched between the circuit boards, and the circuit boards are bonded with an epoxy resin sealant to hold a gap of specified size between the circuit boards. The liquid crystal is then injected into this empty cell.
To manufacture polymer dispersion liquid crystal panels using this manufacturing method, it is sufficient to use a UV-setting resin, e.g., an acrylic resin, for the polymer matrix material. This is because the resin exists as a relatively low viscosity precursor of monomers and/or oligomers before injection, and the liquid crystal blend (the liquid crystal solution) has sufficient fluidity for injection at room temperature. As a result, the manufacturing method of a conventional liquid crystal panel can be used to produce the circuit board laminate to which the liquid crystal solution is then injected. After injection, the panel is exposed to light to advance the setting reaction forming the polymer dispersion liquid crystal layer.
By irradiating the panel with ultraviolet light after polymer injection, a polymerization reaction is effected only in the resin components to form the polymer, and the liquid crystal components are phase separated. When the liquid crystal content of the solution is less than the resin content, grain-shaped liquid crystal drops are separately formed; when the liquid crystal content is greater, the polymer matrix exists in a granular or networked state in the liquid crystal material, and the liquid crystal is formed in a continuous layer. The size of the liquid crystal drops or the hole size in the polymer network at this time must be fairly uniform and within the range of approximately 0.1 .mu.m to several .mu.m, otherwise light scattering performance will be poor and contrast low. As a result, the material must be completely curable in a relatively short period of time. UV-setting resins satisfy these requirements and are therefore preferable.
The operation of a polymer dispersion liquid crystal is described briefly below with reference to FIGS. 27 and 28. Shown in FIGS. 27 and 28 are the array substrate 231, pixel electrode 232, counter electrode 233, drop-shaped liquid crystals 234, polymer 235, and counter electrode substrate 236. Note that thin-film transistors (TFT) and other components not shown in the figures are connected to the pixel electrode 232, and light is modulated by changing the direction of liquid crystal orientation on the pixel electrode by turning the TFT on/off to apply a voltage to the pixel electrode.
When a voltage is not applied as shown in FIG. 27, the drop-shaped liquid crystals 234 are oriented in irregular directions, causing a difference in the refractive indices of the polymer 235 and the drop-shaped liquid crystals 234, and scattering to the incident light.
When a voltage is applied to the pixel electrode 232 as shown in FIG. 28, the liquid crystals become oriented in the same direction. If the refractive index of the liquid crystals when oriented in a specific direction is adjusted to match the refractive index of the polymer, the incident light will not be scattered and will be emitted from the array substrate 231. Note that when the liquid crystals have a drop-like shape similar to a PDLC, the average diameter of the drop-shaped liquid crystal is called the average particle diameter. In a PNLC the hole diameter is normally expressed, but this is also referred to as the average particle diameter in this specification.
An example of a projection display device using this type of polymer dispersion liquid crystal panel is described in U.S. Pat. No. 5,150,232. In the device described in this patent, the light from a single light source is split into the wavelength groups of the three primary colors (red, (R), green (G), and blue (B)) conducted to different optical paths using a dichroic prism; a polymer dispersion liquid crystal panel is placed in each optical path as a light valve; the light modulated by these polymer dispersion liquid crystal panels is then merged again using a dichroic prism; and the image is enlarged and projected to the screen using a projection lens.
In a conventional TN liquid crystal panel, a shading layer called a black matrix is formed in the non-display area between pixels. More specifically, in an active matrix liquid crystal panel comprising switching elements, this black matrix is formed on a substrate opposite the switching elements and signal electrodes. This is to improve contrast, to prevent photoconductivity in the TFT, and to block light leakage caused by reverse tilting of the liquid crystals resulting from horizontal fields applied between the signal lines and the electrodes in an active matrix liquid crystal panel.
However, this black matrix cannot be formed when a UV-setting resin is used for the polymer matrix in a polymer dispersion liquid crystal. This is because if the polymer dispersion liquid crystal panel is formed by the method described above in the empty cell in which the black matrix is formed, the UV light used for UV-setting of the resin will be blocked by the black matrix, and the resin in the shaded are will remain uncured.
In a polymer dispersion liquid crystal panel in which a black matrix is not formed, the field applied between the signal lines and the electrodes causes the liquid crystal molecules to stand up, the scattering power to weaken, and light leakage to occur. The light leaking from between the pixels thus blurs the image, and results in an image lacking in sharp definition. When a TFT is used for switching elements, this leaked light penetrates to the semiconductor layer of the TFT, a leakage current caused by photoconductivity develops, and crosstalk and other display problems occur.
In a projection display device using a reflective liquid crystal panel, if the black matrix is formed using a metallic thin film of chrome or another metal in the liquid crystal panel, the light incident to the liquid crystal panel and reflected by the black matrix is emitted without being modulated in any way, resulting in extraneous reflected light that lowers contrast.
In addition, scattered light with a large emission angle is completely reflected to the liquid crystal layer by the substrate-air interface. Beams returning to the non-display areas between pixels, in particular, induce photoconductivity in the TFT, producing scattering in this area again, after which the scattered light returns to the emission side. The result is reduced display contrast and display quality.
Moreover, the wavelength dependency of the scattering characteristics of the polymer dispersion liquid crystal panel is high. In particular, the scattering characteristic of red light, which has a long wavelength, are inferior to the characteristics of green and blue light. As a result, liquid crystal panels that modulate red, green, and blue light for each pixel by means of color filters suffer from poor contrast in the red spectrum only.